This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Problems of AM”.

1. If the modulating frequency of a carrier wave varies between 700Hz and 7KHz, find it’s bandwidth?

a) 10 KHz

b) 23 KHz

c) 17.3 KHz

d) 12.6 KHz

View Answer

Explanation: Modulating Frequency (f

_{m}) = V

_{max}-V

_{min}, where,

V_{max} = Maximum Amplitude of an amplitude modulated,

V_{min} = Minimum amplitude of an amplitude modulated,

f_{m} = 7KHz – 700Hz = 6.3KHz

Bandwidth = 2f_{m} = 2 x 6.3 = 12.6 KHz.

2. A 400W carrier wave is modulated to a depth of 65%. Find the total power of modulated wave?

a) 512.5W

b) 493W

c) 484.5W

d) 609.6W

View Answer

Explanation: Total power, P

_{t}= P

_{c}(1 +

^{µ2}⁄

_{2}), where P

_{c}= Carrier Power = 400W

where Modulation Index (µ) = 0.65,

so P

_{t}= 400 (1 +

^{0.652}⁄

_{2}). On solving it we get P

_{t}= 484.5W.

3. Inductance and capacitance of a line is 0.8 ^{μH}⁄_{m} and 32 ^{pF}⁄_{m}. Find Z_{0}?

a) 158

b) 166

c) 143

d) 127

View Answer

Explanation: The characteristic impedance or surge impedance (Z

_{0}) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; which is, a wave travelling in one direction in the absence of reflections in the other direction.

.

4. Find VSWR of a line having maximum and minimum value equals to 120mV and 40mV respectively?

a) 3

b) 2

c) 1

d) 4

View Answer

Explanation: The voltage standing wave ratio (VSWR) is defined as the ratio of the maximum to minimum voltage on a loss-less line. In this way, the VSWR is measured at a particular point and the voltage maxima and minima do not need to be determined along the length of the line.

5. A 1000 KHz carrier is modulated with 300 Hz, 8000 Hz and 2 KHz waves. Determine the frequencies whose chances of occurring in output is least?

a) 1000 KHz

b) 1002 KHz

c) 998 KHz

d) 999.2 KHz

View Answer

Explanation: Frequencies present in output are of the form f

_{c}± f

_{m}, f

_{c}± 2f

_{m}, f

_{c}± 3f

_{m}. And 1000 KHz is a multiple of none in the range. Whereas, rest options are one of the multiples in the range.

6. If average power of a transmitter is 4kW and maximum power is 20000KW. What is its duty cycle?

a) 2 x 10^{-3}

b) 2 x 10^{-4}

c) 3 x 10^{-3}

d) 0.05

View Answer

Explanation: Duty cycle is the proportion of time during which a component, device, or system is operated. The duty cycle can be expressed as a ratio or as a percentage.

7. The voltage and electric current of a line are respectively 5kV and 1000μA. Find its power?

a) 12W

b) 13W

c) 5W

d) 10W

View Answer

Explanation: Electrical power is the rate at which electrical energy is converted to another form, such as motion, heat, or an electromagnetic field.

Power, P = V x I. By substituting the values of voltage(V) and current(I), we have P = (5×10

^{3})(1000×10

^{-6}) = 5W.

8. ________ microphones works on piezoelectric effect.

a) Crystal

b) Carbon

c) Moving coil

d) Condenser

View Answer

Explanation: Piezoelectric effect is the ability of materials to generate an electric charge in response to applied mechanical stress. Crystals which demonstrate the piezoelectric effect produce voltages when they are deformed. The crystal microphone uses a thin strip of piezoelectric material attached to a diaphragm. The two sides of the crystal acquire opposite charges when the crystal is deflected by the diaphragm.

9. If modulation index of an AM wave is increased from 1.5 to 2, then the transitted power ________

a) remains same

b) increases by 20%

c) increases by 41%

d) increases by 50%

View Answer

Explanation: When m=1.5, transmitted power

.

= 0.41 x 100

= 41%

Therefore, there is an increase in total power by 41%.

**Sanfoundry Global Education & Learning Series – Analog Communications.**

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